class Solution {
public:
    void reverseString(vector<char>& s) {
        int first=0;
        int end=s.size()-1;
        char ch;
        while(first!=end&&first<end)
        {
            ch=s[first];
            s[first]=s[end];
            s[end]=ch;
            first++;
            end--;
        }
    }
};
